1. Technical Field
This invention relates generally to a catalyst for treating products of combustion, and more particularly to such a catalyst that both reduces various oxides of nitrogen (NO.sub.x) and oxidizes carbon monoxide (CO) components of gaseous products of combustion.
2. Background Art
Atmospheric NO.sub.x, which primarily consists of nitric oxide (NO) and nitrogen dioxide (NO.sub.2), results in air pollution as well as the production of acid rain and urban smog. More specifically, nitrogen dioxide is linked as a causative agent in bronchitis, pneumonia and alteration of the immune system in humans. NO.sub.x also participates in the formation of ground-level ozone in the presence of sunlight. Industrial and other combustion processes, e.g., internal combustion engines, produce large amounts of NO.sub.x, which is a severe threat to the environment.
About one-half of all man-made NO.sub.x can be attributed to automobile engine emissions. Recently enacted federal and state regulations impose stringent restrictions on auto engine emissions. Future regulations now being proposed will place even tighter restrictions on engine emissions.
Conventionally, noble metals such as platinum and rhodium, supported on alumina, are used for controlling auto engine emissions. The performance of noble metal based catalysts is generally improved by increasing the loading of the noble metal, which is very expensive. A vanadium-based catalyst is now being used in power plants, but poses an environmental threat due to the production of volatile vanadium compounds. More recently, the use of ammonia (NH.sub.3) has been demonstrated as the reducing agent in selective catalytic reduction (SCR) of NO.sub.x, but use of ammonia in a mobile system may not be feasible. The use of metal substituted zeolites as candidates for NO.sub.x reduction has also been proposed. It has been found that excessively ion exchanged copper zeolites can reduce 80% of NO to nitrogen, but the decomposition is inhibited in the presence of high concentrations of oxygen such as that typically present under "lean burn" conditions required for optimum fuel economy in automobile engines. It is also known that catalysts comprised of copper zeolites, in the presence of hydrocarbons, can reduce NO under oxygen-rich conditions. In this arrangement, gasoline has been introduced, as the hydrocarbon source, into the exhaust gas mixture to optimize the performance of the catalyst. This practice reduces overall fuel economy. Catalysts in current use based on platinum or rhodium are costly, do not work under lean-burn conditions, and will not be able to meet future emission standards.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a catalyst system that is economical to produce and is capable of effectively treating NO.sub.x emissions in automotive exhaust systems. It is also desirable to have such a catalyst system that does not require the use of platinum or rhodium or other rare metals. Furthermore, it is desirable to have an inexpensive catalyst that, under lean burn conditions, not only reduces NO.sub.x but also oxidizes carbon monoxide and hydrocarbons. Still further, it is desirable to have such a catalyst that reduces NO.sub.x under oxygen-rich conditions and is not dependent on hydrocarbon for its activity.